1. Field of the Invention
The present invention relates to a magnetic disk unit and a sector control method for a magnetic disk unit.
2. Description of the Related Art
In recent years, marked progress has been made in a compact magnetic disk unit technologies. The performance and storage capacities of compact magnetic disk units have been considerably improved.
The major factors contributing to the improvement in the performance are reduced seek time, reduced rotation wait time due to the high-speed rotation of the disk, and an improved data transfer speed due to the increased recording density. These factors result from the improvements in fundamental magnetic disk unit technologies.
Accordingly, the overhead attributable to internal processing, which had usually been disregarded in the past, has become a critical problem. It is now required that the internal processing speed is reduced to the order of microseconds.
In recent years, disks having a diameter of 3.5" or less has become standard in an effort to cope with an increasing demand for a small-sized magnetic disk unit. When it comes to disks having the same diameter, the contradictory qualities of a large storage capacity and a small height are required.
A technique often employed for increasing a storage capacity per magnetic disk is to divide a magnetic disk into several concentric zones, and to write data using a different recording frequency in each zone.
The above technique is generally referred to as a constant density recording (CDR) technique or a zone density recording (ZDR) technique.
According to a conventional technique for writing data at a single recording frequency, the recording density along the outer track of a disk is lower than that along the inner track thereof. In contrast, according to the CDR or ZDR technique, the recording frequency along the outer track is made higher than that along an inner track. In this way, the recording densities along the outer track is substantially the same as that at the inner track, thus offering a large storage capacity.
In a conventional magnetic disk unit, index pulses and sector pulses are produced according to servo information previously written on a servo surface of a magnetic disk (which will be referred to as the medium). These pulses are used to control formatting. This technique is implemented in a known hard-sectored magnetic disk unit.
This kind of magnetic disk unit has a microcomputer responsible for internal control and a format control unit to control formatting.
At present, in a hard-sectored magnetic disk unit, when a head reaches a sector (sector n-1) immediately preceding a sector (sector n) at which reading, writing, or formatting is to start, a microcomputer issues a command to a format control unit so that the format control unit is activated synchronously with the leading edge of a sector pulse.
Some means is therefore needed to detect the sector immediately preceding the sector at which processing is to start, and to activate the format control unit within a time interval corresponding to the length of one sector after the sector is detected.
The simplest method of detecting a specific sector is to poll a sector counter using a microcomputer. However, when engaged in polling, the microcomputer cannot accept an interrupt. An interface in a host computer must wait until polling is completed, and ecessive overhead therefore arises. As a solution of this problem, a technique using hardware to detect a specific sector and output an interrupt signal has been widely adopted in the past.
A hardware-based method for detecting a specific sector and outputting an interrupt signal to a microcomputer will be described. A magnetic disk unit receives a command from a host computer to access a disk.
When the head is to start reading or writing at a sector n, the microcomputer instructs a specific sector detector to detect a sector n-1. Namely, the microcomputer activates the interrupt circuit.
Thereafter, in response to an interrupt signal generated, the microcomputer issues a command to a format control unit. Namely, the microcomputer activates the format control unit.
Consequently, when the head reaches the given sector n, the format control unit is activated. An operation to read or write data is then carried out.
That is to say, the time interval after an interrupt occurs to access a sector n-1 until a command is issued to a format control unit must not exceed the time interval corresponding to the length of a single sector. This interruption for sector access is therefore given top priority. Consequently, Another interrupt request cannot be accepted during the sector access operation.
As far as the above procedure is concerned, the time interval of sector access operation is seen as an overhead by an interface in a host computer.
In the CDR or ZDR technique, when a magnetic head moves over a zone, it takes considerable time to identify data in a sector header indicating a destination sector. An excessive wait time arises every time a head overpasses a zone, and a technology therefore been suggested such that when a head moves, on efficient and quick identification of a bit position of data in a sector header indicating a destination sector is possible.
As mentioned above, a compact magnetic disk unit generally adopts a fixed sector length technique. This technique makes it necessary to output a sector pulse serving as a delimiter between sectors to a hard disk controller (HDC). The sector pulse is output at certain intervals (at intervals of a sector length) along a track of a medium with an index mark formed on the medium as an origin.
In general, a servo clock is a clock whose pulses are counted in order to define a sector length and which attains synchronism with a rotational variation of a disk. When a single recording frequency is used to write data, a count of servo clock pulses corresponding to a sector length is constant between tracks on a disk. Therefore, when a disk is activated, the sector length is merely be set in a load register for a counter.
Thereafter, the sector length is re-loaded in response to every index mark or sector. Counting is then performed. A sector pulse is produced in response to a Carry Out signal provided by a counter.
When the aforesaid CDR technique is employed, the number of servo clock pulses corresponding to a sector length differs from zone to zone. When a magnetic head moves to a new zone, a bit position of data in a sector header indicating a destination sector cannot be identified until the counter is reset to zeros in response to an index mark. Therefore, every time the head moves to a new zone, an index wait time is needed.